The present invention relates to a method of growing Si epitaxial films.
Recently, it has become impossible to realize super semiconductor devices of the next generation with an ultra-high integration degree and an ultra-high speed by merely miniaturizing conventional semiconductor device structures. Therefore, it is essential to develop new device structures.
A method in which an automatically positioned pattern is formed in a specific region on a substrate without using any lithographic techniques is called self-alignment. This self-alignment is a crucial technique in the fabrication of semiconductor devices, since it not only can simplify the fabrication process steps but also gives a large degree of freedom in designing fine device structures.
This self-alignment technique is also widely used in selective growth of Si epitaxial films, which is the object of the present invention. This technique has the advantages of being able to vary the impurity distribution of an active region and increase the degree of freedom in process design. In recent years, therefore, attempts using the techniques for selectively growing Si epitaxial thin films have been made vigorously in searching for and effectuating new device structures. As an example, high-speed Si bipolar transistors having a very thin base layer have been studied and developed extensively. As a technique for forming this base layer, a selective growth method which employs gas source molecular beam epitaxy using silane gas as an Si Source has been proposed (e.g., H. Hirayama et al., "Journal of Applied Physics," 1988, Vol. 52, pp. 2242 and 2243).
By using the fact that the Si surface is more chemically active than the SiO.sub.2 surface in an ultra-high vacuum, this method causes silane gas to be dissociatively adsorbed only on the Si surface. Hydrogen atoms in the silane gas molecules thus dissociatively adsorbed are desorbed by heat, so only Si atoms remain on the substrate. This makes epitaxial growth of Si feasible. The result is selectivity by which a Si film is epitaxially grown only on the Si surface and no Si is grown on SiO.sub.2 provided that the growth time is within a predetermined time period.
Consequently, the use of this method simplifies the fabrication process and makes it possible to form fine base-emitter junction portions with a high accuracy. Additionally, since a gas in the molecular beam state is used as the Si source, a high film thickness controllability can be attained.
In the above selective growth method using the gas source molecular beam epitaxy, however, the SiO.sub.2 surface is not perfectly inactive. Therefore, if the time of selective growth is prolonged, dissociative adsorption of silane gas gradually takes place on the SiO.sub.2 surface as well as on the exposed Si surface, and nucleation of Si proceeds on the SiO.sub.2 surface. Eventually, poly-Si is deposited on that surface and the selectivity collapses. In other words, the use of this selective growth method imposes a limitation on the thickness of Si epitaxial films that can be selectively grown.
Moreover, the nucleation rate on the SiO.sub.2 surface abruptly increases at temperatures of about 550.degree. to about 800.degree. C. at which a good crystallinity can be obtained and the thermal influence on device structures is insignificant. To obtain selectivity, therefore, it is necessary to largely decrease the supply amount of silane gas as an Si supply source per unit time, and this greatly decreases the growth rate. Consequently, the thickness of Si epitaxial films which can be selectively grown is further decreased.
These disadvantages discussed above are serious problems in using the selective growth method employing the gas source molecular beam epitaxy in the semiconductor fabrication processes.